Current limiting elements, or when using the terminology of the art, short circuiting protectors are mainly comprised of fuses and circuit-breakers which most often possess current limiting properties. The technique is known to the art and several standards, such as IEC 269 concerning fuses, and
IEC 947-2 concerning circuit-breakers, have been instituted. The short-circuiting protector is excited by the short circuiting currents flowing therethrough. The shortcircuiting protector is excited in accordance with two main principles and is therefore divided here into the following groups 1 and 2:
1. Fuses, thermistors with positive temperature coefficients and self-restoring short-circuiting protectors described in U.S. Pat. No. 3,886,551 are excited when short-circuiting currents flow therethrough as a result of the increased ohmic power development in the protector. When the applied electrical energy has caused a temperature increase in the protector corresponding to the melting point of vital material in the protector an increase in resistance occurs and limitation of the short-circuiting current begins. PA1 2. Arc-based, current limiting cut-outs, for instance circuit-breakers are excited directly, through the conversion of magnetic energy to mechanical energy, by electrodynamic current forces occurring on the electrical contact system included in the circuit-breaker, or indirectly through the medium of a separate excitation device comprised of an electromagnetic release device, a so-called "plunger or schlagstiftanordnung", which is also excited by the main current. An armature included in a magnetic circuit acts on the electrical contact system and/or on a spring mechanism release device which performs an on/off-function. Remote control is also used, for instance in contactors, for maintaining two stable mechanical states of equilibrium, on and off respectively. Electrical contact systems in which electrodynamic current forces act directly on the electrical contacts are earlier known to the art, for instance from Patent Specifications GB 1,519,559, GB 1,489,010, GB 1,405,377. PA1 a) Considerably increased sensitivity at high current derivatives and short-circuit currents due to a resilient pressure device and preferred electrode configuration, which together with the particularly configured electrically conductive elastomeric body will repeal the electrodes. PA1 b) The device can be made very low-ohmic, because of deformation of the contact transition between electrically conductive elastomeric body and electrode. PA1 c) A smaller selectivity problem in electric circuits which include main and subordinate protectors. PA1 d) The element returns to its initial resistance after passing from a low-resistive state to a high-resistive state. PA1 e) A simple circuit breaking device which may possibly not require the provision of arc shields when the electrodes are connected mechanically to a conventional on/off mechanism for circuit-breakers which in the on-position maintain the requisite pressure between electrode (=contact) and electrically conductive elastomeric body. PA1 f) Eliminated welding risk when a circuit-breaker arrangement according to point e) above is included. PA1 g) A vibration-insensitive and rebound-insensitive switch-on function. PA1 h) The possibility of adjusting the sensitivity of the device when the pressure maintained by the pressure device can be adjusted and varied in a known manner, thereby enabling one and the same overload protector to be used in an extended rated current range. PA1 i) Very small external dimensions, since the electrically conducting elastomer material can be given a very low resistivity &lt;1 mohmcm. PA1 j) The provision of exclusive chokes in thyristor circuits can be avoided.
Hybrids in which the two principles are used are disclosed in Patent Specification GB 1,472,412 and in the article "A New PTC Resistor for Power Applications" by R. S. Perkins, et al, published in the journal IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-5, No. 2, June 1982, pages 225-230 and publications U.S. Pat. No. 3,249,810 and DE 35 446 47, among others.
One serious drawback with short-circuiting protectors according to groups 1 and 2 above, particularly in the case of high and steep (=rapidly growing) short circuiting currents, resides in the high intrinsic inertia. Thermal inertia has a limiting effect on the short-circuiting protectors described under group 1 above whereas in the case of arc-based circuit-breakers it is the mechanical inertia, i.e. the mass inertia, which becomes significant when wishing to separate the electrical contacts quickly. As a result of the mass inertia, the arc is delayed on the electrical contacts in arc-based circuit-breakers, and consequently the arc voltage, important in achieving current limitation, will not reach the values at which the otherwise monotonously growing short circuit current is limited until a relatively long delay time (ms) has lapsed. Furthermore, a very high contact pressure, proportional to the square of the rated or nominal current of the apparatus, is required in order for the electrical contacts to be able to carry rated current under normal operating currents. This also prevents the electrical contacts from separating quickly, since the contact pressure is opposed to the electrodynamical repelling and separating forces.
The possibility of adjusting the sensitivity of the short-circuiting protectors described under categories 1 and 2 above is highly limited. Consequently, there is required a comprehensive coordination work with main and subordinate protectors included in electric circuits. Standards have therefore been worked-out, for instance DIN 57636 Teil 21/VDE 0636 Teil 21 .sctn. 7,12 and IEC 947-2, since erroneous coordination may, among other things, incur selectivity problems which are difficult to rectify (adjust) in existing systems.
As a result of the aforesaid drawbacks, and in particular inertia, short-circuiting protectors based on the principles disclosed in categories 1 and 2 above are less suited as short-circuiting protectors or current transient protectors for thyristors or electronic equipment, since they are sensitive to both high current derivatives and high short-circuiting currents can also occur in capacitive circuits or inductive motor circuits with high presumptive short-circuiting currents. Typical values of presumptive short-circuiting currents are Ik=50-100 kA and corresponding current time derivatives from 22-44 kA/ms. With a rated current of 100 A, a conventional fuse will then allow a current peak of about 16 kA and .intg. i.sup.2 .multidot.dt.apprxeq.20 kA.sup.2 s to pass through, which greatly exceeds the permitted values of corresponding thyristors. Consequently, chokes are often included in thyristor circuits in order to reduce current derivatives, therewith enabling the aforedescribed short-circuiting protector to be used.
A self-restoring short-circuiting protector is mainly comprised of so-called thermistors. The expression PTC-element is an accepted designation of thermistors whose resistivity has a Positive Temperature Coefficient.
Electrically conductive polymer compositions, particularly PTC-compositions, and devices in which PTC-compositions are included are known to the art. Reference in this regard can be made to U.S. Pat. Nos. 2,978,665, 3,351,882, 4,017,715, 4,177,376 and 4,246,468, and also to U.K. Patent No. 1,534,715. Later developments are described, for instance, in German Patent Nos. 2,948,350, 2,948,281, 2,949,174 and 3,002,721, and also in various Patent Applications, such as U.S. Ser. Nos. 41,071 (MPO 295), 67,207 (MPO 299) and 88,344 (MPO 701), and patent applications such as U.S. Ser. Nos. 141,984 (MP=712), 141,987 (MPO 713), 141,988 (MPO 714), 141,989 (MPO 715), 141,991 (MPO 720) and 142,054 (MPO 725).
One problem with PTC-elements is that when heated by the current flowing therethrough and the temperature is reached at which the PTC-elements become self-adjusting, the voltage is taken over by a fragment of the PTC-element and the fragment is subjected to very high stresses, which are liable to destroy the PTC-element. PTC-embodiments in which this problem is eliminated are known, for instance, from European Patent EP 0,038,716. PTC-elements for overload protectors are often constructed of a polymeric material, for instance high-pressure polyethylene, Containing particles of an electrically conductive material, for instance lamp black or carbon black, and exhibit a resistivity with high positive temperature coefficient.
Ceramic thermistors which exhibit PTC-characteristics are known from Patent Publication GB-A-1,570,138. The most common ceramic thermistors are based on BaTiO.sub.3 or V.sub.2 O.sub.3.
One advantage afforded by the polymer-based thermistor in comparison with the ceramic thermistor is that its resistance increases monotonously with temperature. It is also relatively cheap to produce. However, commercially available polymer-type thermistors are designed for relatively low rated or nominal voltages and cannot therefore be used readily in distribution networks for instance. Furthermore, the configuration and electrode connections of the thermistors are normally such that the thermistors are subjected to large repulsion forces at high short-circuiting currents, as a result of anti-parallel current paths, therewith tearing the electrodes apart. It is also known that sandwich-type, polymer-based PTC-elements do not return to the initial resistance after passing from a low resistive state to a high resistive state. In more serious cases, when the PTC-elements are subjected to very high electrical stresses, such as short-circuiting currents, bubbles and cracks form in the central parts or in other parts of the polymer composition of the PTC-element, so that the element will no longer function, i.e. the element is destroyed.
For these reasons, polymer-based thermistors have not hitherto been used to any appreciable extent in practice within electric power technology, but have mainly only been used to protect electronic equipment, although the thermal inertia limits the fields of possible application.
An essential difference between thermistors and fuses is that thermistors will self-restore after a short-circuit, i.e. thermistors can be reused after a short-circuit, which also applies to circuit-breakers.
Elastomers are comprised of all polymers that exhibit elastic properties which are similar to those exhibited by natural rubber. Elastomers can be compressed or stretched within a relatively large permitted elastic area, and return to their original state when the load is removed. Electrically conductive elastomers are a class of rubber and plastics which have been made electrically conductive, either by the addition of metal mixtures or by orientating metal fibres under the influence of electric fields, or by the addition of different carbon mixtures or ceramics, for instance V2O3-material dispersed in the manner described in the article "V2O3 Composite Thermistors" by D. Moffat, et al, published in Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics, 1986, pages 673-676. In rubber, there, is used several types of "carbon black", for instance graphite, acetylene black, lampblack and furnace black with particle diameters ranging from 10-300 nm. Examples of appropriate rubber materials which become electrically conductive after adding metal mixtures or carbon mixtures are butyl, natural, polychloroprene, neoprene, EPDM, and the most important silicone rubber. Additives of metals and metal alloys in powder form suited as elastomer additives are silver, nickel, copper, silver-plated copper, silver-plated nickel, and silver-plated aluminium.
Electrically conductive elastomer are used as pressure transducers within transducer technology. The electrical properties are changed when electrically conductive elastomers are deformed, for instance as a result of being subjected to pressure or tension, which manifests in a change in resistance.
The most common types of carbon or metal-filled plastics are polyethylene and polypropylene. These are used at present for heating cables and for overload protectors, for instance the earlier mentioned polymer-based PTC-thermistors.
However, the inclusion of an electrically conductive filler impairs the mechanical properties of the plastic. The material becomes brittle and hard and is therewith not readily deformed. These materials are therefore unsuitable as pressure transducers and also require a relatively complicated contacting technique for PTC-applications. A further limitation of carbon-filled plastics resides in their relatively high resistivity, which is typically one 1 Ohmcm and higher. On the other hand, metal-filled plastics can be produced with significantly lower resistivity, lower than 0.5 Ohmcm, although voltage or tension stability becomes very poor, and consequently these materials are not suited as overload protectors.
Electrically Conductive elastomers can be given very low resistances, for instance resistances of 2 mOhmcm or lower, by admixing metal powder. One advantage afforded by elastomers is that they are very soft in comparison with carbon-filled polyethylene and polypropylene, even when containing large quantities of electrically conductive filler. Such elastomers will have a typical Shore number of between 20-80, according American Standard ASTM D2240 (Q/C).